A theoretical study of the vibrational spectrum of maleimide

B3LYP and MP2 theoretical calculations with 6-31++G(d,p) basis set have been performed to interpret the infrared spectrum of maleimide. The anharmonicity effect has been included to correct the calculated harmonic frequency values. As expected, this has allowed a better agreement with the experimental ones, especially B3LYP values. The more pronounced changes have been verified to the X H (X = C or N) stretching modes. The calculated IR intensities compare favorably well with their corresponding relative experimental values in vapor phase. The large discrepancy is found to the symmetric C C stretching mode of the heterocyclic ring. It appears to be a strong band at 897 cm −1, in contrast to B3LYP and MP2 calculated values. The calculated intensities due to the out-of-plane and in-plane bending and stretching modes for the X H oscillators (X = C or N) have been adequately interpreted in terms of hydrogen atomic charges and X H charge-fluxes extracted from the modified charge–charge flux-overlap model (CCFOM) for infrared intensities.